1. Field of the Invention
The present invention relates to the oxidation of wastes. In particular, the present invention relates to an apparatus and method for two-stage oxidation of wastes. The United States Government has rights in this invention pursuant to Contract No. DE-AC0989SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Many industrial processes generate wastes in the form of aqueous waste streams, sludge solids, aqueous supernate, incinerator ash, incinerator off-gas condensate, and so forth. The cost of disposal in landfills, once the most economic method for getting rid of wastes, has greatly increased because of increased documentation and permitting requirements, transportation costs and handling costs, and has acquired a considerable contingency cost because of uncertainties about the advisability and durability of land disposal of wastes.
Many types of waste materials can and are detoxified before disposal. Wastes that contain hazardous organics such as halogenated hydrocarbons, dioxins, phenols, and so forth present significant environmental disposal problems unless they are detoxified. Incineration only partly destroys the hazardous constituents of many wastes. In addition, volatile organics may escape into the atmosphere. Byproducts of incineration include bottom ash, aqueous incinerator offgas condensate (blowdown), or mixtures of ash and offgas condensate, all of which may contain residual hazardous substances.
Electrochemical oxidation processes are also used to treat wastes. Thus, Kenney, et al. (U.S. Pat. No. 4,582,613) add copper (II) oxide to an aqueous feed stream containing toxic materials such as cyanide, hydrocarbons, halogenated hydrocarbons, and dioxins. The stream is reacted under elevated pressure and temperature conditions to oxidize the toxic materials to less toxic or innocuous compounds.
Silver is a known agent for oxidizing organic compounds. Kitahara, et al. (U.S. Pat. No. 4,910,001) disclose a composition for cleaning a gas that contains a toxic component such as arsine, phosphine, monosilane, diborane, and hydrogen selenide. The composition comprises pellets formed of manganese dioxide and cupric oxide, having deposited thereon a silver compound such as silver (I) oxide or silver (II) oxide. Steele (U.S. Pat. No. 4,874,485) adds organic wastes to an aqueous electrolyte comprising nitric acid and containing Ag(I) as a primary oxidizing species. The electrolyte is heated to a temperature above 50.degree. C. to produce Ag(II) ions which decompose the wastes. Zabolotny, et al. (U.S. Pat. No. 3,793,171) place a gas stream that carries oxidizable impurities in contact with an aqueous acid stream containing an electrolytically regenerable oxidizing agent (Ag(II)). After a sufficient contact time to remove oxidizable impurities from the gas stream, the gas stream is removed from the contact area and vented or further treated as desired. The aqueous acid stream is removed from the contact area, the oxidizing agent is regenerated, and the cycle is repeated.
Ionic oxidizing agents, particularly Ag(II) ions, are known to oxidize many organic and inorganic compounds rapidly and effectively. However, electrochemical oxidation using Ag(II) is not cost-effective due to the large amounts of power required to produce the Ag(II) ions. Treating many types of wastes using currently available technology is expensive and requires a substantial commitment of resources. There is a need for a simple, relatively cost-effective way to treat oxidizable wastes.